直方图 Histogram是一种特殊的列统计信息，详细描述了目标列的数据分布情况。存储在数据字典基表 histogram$;
专门为了准确评估分布不均匀的目标列的可选择率、结果集的cardianlity。
详细描述了目标列的数据分布情况，并将分布情况记录在数据字典里。
select t1.column_name name,t1.num_distinct "#dst",t1.low_value,t1.high_value,
t1.density dens,t1.num_nulls "#null",t1.avg_col_len avglen,t1.HISTOGRAM,t1.num_buckets "#bkt"
from user_tab_col_statistics t1
where t1.table_name='T';
NAME #dst LOW_VALUE HIGH_VALUE DENS #null AVGLEN HISTOGRAM #bkt
------------------------------ ---------- ---------------------------------------------------------------- ---------------------------------------------------------------- ---------- ---------- ---------- --------------- ----------
ID 1000 C102 C20B .001 0 4 NONE 1
VAL1 457 C105 C22160 .002353264 481 3 HEIGHT BALANCED 254
VAL2 6 C20202 C20207 .0005 0 4 FREQUENCY 6
VAL3 6 C20202 C20207 .0005 0 4 FREQUENCY 6
PAD 1000 203F2C5D523371586E695D456775533C565A522A4F3128234F43502F77353026 7E7E7438735D7A464E77205B7B79454D517E384069784521344735457E2F2120 .001 0 251 HEIGHT BALANCED 254
select utl_raw.cast_to_number(low_value) as low_value,
utl_raw.cast_to_number(high_value) as high_value
from user_tab_col_statistics
where table_name='T'
and column_name='VAL1';
/*
LOW_VALUE HIGH_VALUE
---------- ----------
4 3295
*/